Scrape off as much of the plastic as you can with a butter knife. Avoid using a sharp blade so you won't damage the heating element. A butter knife works as a lever to pry off the plastic.
Fill the bottom of the dishwasher with white vinegar so the heating element is completely covered. Let it sit overnight so the acidity of the vinegar can work on any remaining plastic on the element. Run the dishwasher empty the next morning with the vinegar still in the basin. The heating element should heat normally, and any remaining plastic will be flushed out.
Open the dishwasher mid cycle and use the butter knife to remove any remaining plastic, which should now be softer and more pliable.
Finish the wash cycle to flush out the vinegar and any small particles of plastic that may still be in the dishwasher.